The major objective of this proposal is description and characterization of a metabolic pathway for the synthesis of urea and disposal of ammonia in the mammalian organism which is an alternative to the classic Krebs-Henseleit (K-H) urea cycle. There exist several patients with various enzzmatic blicks in the K-H cycle in whom unexplained normal or near normal urea synthesis occurs. We have speculated that an alternate cycle, utilizing the homologous compounds of the K-H cycle, might exist, by which ammonia could be converted to urea from the transcarbamylation of lysine, followed by the formation of homocitrulline, homoargininosuccinic acid, homoarginine, and finally urea. Such a pathway, ordinarily of little or no physiological significance, might become enhanced or more prominent in individuals with enzymatic deficits in the primary urea cycle. We have obtained data which strongly suggest that such a cycle does exist in the rat liver. We propose to explore the specific reactions and characteristics of the K-H and alternate urea cycle in normal human liver, fibroblasts, and lymphocytes; in fibroblasts and lymphocytes obtained from a citrullinemic patient and his parents; and in liver from various lower species. The activity of the 2 cycles will be compared through the use of specifically labeled precursors ((urdido) minus C14 minus citrulline and H3 minus homocitrulline) and observations of their relative incorporation into urea and intermediate compounds (homocitrulline, homoargininosuccinic acid, homoarginine). We propose to test if the alternate cycle in liver can be made more prominent by exposure to high protein or ammonia loads in adult animals. The long-term effects of early exposure to elevated protein or ammonia levels will be studied by analysis of hepatic enzyme activities in animals treated during infancy. Finally, we will attempt to influence the selection and ultimate prominence of the alternate pathway by manipulation of the media surrounding growing tadpoles during the period of their metamorphosis, in which sequential development of the primary urea cycle enzymes occurs.